1. Field of the Invention
The present invention relates to an apparatus for optical disc spin-coating and a method of manufacturing the same, and more particularly, to an apparatus for optical disc spin-coating formed such that even when the cap is eccentrically placed on an optical disc a cap can be easily attached to and detached from an optical disc by using vacuum pressure, and a method of manufacturing the same.
2. Description of the Related Art
Optical discs are used in optical pickup apparatuses for recording/reproducing information. Examples of optical disc include compact discs (CDs) with a storage capacity of 600 to 800 MB and digital versatile discs (DVDs) with a storage capacity of 4 to 10 GB. Recently, to store more data and achieve higher audio and video quality, blu-ray discs (BDs) or HD-DVDs with a storage capacity of 20 GB or greater have been developed using a 405 nm blue laser technology.
To increase recording density of optical discs, various methods are used. One possibility in this regard is to minimize the size of a light spot, which is achieved by controlling the wavelength of a laser and the number of apertures of a lens according to the following equations:D∝1.22λ/NA  (1)F∝λ/NA2  (2)f∝A/2NA  (3)where D is the diameter of a spot, λ is the wavelength of a laser, NA is the number of apertures of a lens, F is a focal depth, f is a focal distance, and A is the diameter of a lens.
As shown in equation 1, when the wavelength of a laser decreases and the number of the apertures of a lens increases, the size of the spot decreases, the pit of a disc and the size of a corresponding track decreases, and the record density increases inverse-proportionally to the square of the size of the spot. On the other hand, as shown in the equations 2 and 3, when the wavelength decreases and the number of the apertures increases, the focal depth decreases and the focal distance decreases.
That is, the BD has a light spot of a smaller diameter, smaller focal depth, and smaller focal distance than the DVD, which has a light spot of smaller diameter, smaller focal depth, and smaller focal distance than the CD. As the focal depth and the focal distance decreases, the reproducing characteristics become more dependent on the state of the light incidence surface of the optical disc. Accordingly, the incidence surface must be protected from scratches and variance of the thickness of the optical disc must be very small.
Meanwhile, in a method of manufacturing an optical disc, a light-transmitting layer, a protective layer, a lacquer layer, and the like are formed by spin coating. The use of spin coating brings about many advantages. For example, a resin that is removed after the spin coating can be re-circulated in the apparatus, and by controlling the time for the spin coating and the viscosity of the resin the light-transmitting layer and the like can have various thicknesses.
FIG. 1 is a graph illustrating the thickness of the light-transmitting layer with respect to the distance between the center of a substrate and a position at which a photocurable resin is discharged onto the substrate. Referring to FIG. 1, the distance varies from 5 to 25 mm and is increased by 5 mm. Numeral ‘31’ denotes the case where the distance is 5 mm, numeral ‘32’ denotes the case where the distance is 10 mm, numeral ‘33’ denotes the case where the distance is 15 mm, numeral ‘34’ denotes the case where the distance is 20 mm, and numeral ‘35’ denotes the case where the distance is 25 mm. As illustrated in FIG. 1, as the position at which a photocurable resin is discharged onto the substrate is closer to the inner circumference of the substrate, the variance of the thickness of the light-transmitting layer decreases. When the discharge position corresponds to the center of the substrate, theoretically, a light-transmitting layer with no thickness variance can be obtained.
In a method of manufacturing a CD, a recording layer and a reflecting layer are formed on a polycarbonate substrate with a thickness of 1.2 mm by sputtering, and then a thin lacquer layer is formed thereon by spin coating to protect the recording layer, reflecting layer, and the like because the focal distance of a laser is too long. Since the thickness of the lacquer layer is as small as 3 to 5 μm, even when a thickness variance occurs, the variance is very low. In addition, a recording or reproducing light enters from the lower portion of the polycarbonate substrate so that even when the thickness of the upper most layer, that is, the lacquer layer, varies, no errors occur during data reproducing. Accordingly, there is no need to discharge the photocurable resin at the center of the optical disc when the lacquer layer is formed by spin coating.
However, in a method of manufacturing a BD with higher integration capacity using a blue laser, since the focal distance is very short while the integrity of data increases, a reflecting layer, a recording layer, and the like are formed on a 1.1 mm thick polycarbonate and then a 0.1 mm thick light-transmitting layer, through which a reproducing light enters, is formed thereon. Accordingly, the reproduction characteristics of the BD are very dependent on the state of the surface and thickness variance of the photo-transmitting layer.
The light-transmitting layer can be formed by attaching a 0.1 mm thick light-transmitting sheet made of polycarbonate using a reduced pressure adhesive or an ultraviolet curable adhesive. In this case, however, a disc is attached to a large sheet and the remaining part is removed, so that the much of the sheet is wasted, the manufacturing costs are increased, and the environment load is increased. Due to these problems, the spin coating is mainly used for the formation of the light-transmitting layer.
As described with reference to FIG. 1, when the spin coating is performed by discharging the photocurable resin circularly at a predetermined position departing from the center of the optical disc, the thickness of the resin layer increases from the center of the optical disc to the outside. When such an increase of the thickness occurs in the BD, data reproducing errors can occur. In order to prevent this problem, the photocurable resin must be discharged at the center of the rotating disc. However, since a conventional optical disc has a hole at its center, another problem occurs, i.e., the photocurable resin can leak into the hole. As a result, many techniques have been developed to prevent the leakage of the photocurable resin into the hole.
For example, according to Japanese Patent Laid-open Publication No. 1998-289489, a center hole of an optical disc is covered by a cap, and then the cap is detached using an electromagnet when the spin coating is completed. In this case, however, when the cap is eccentrically placed on the optical disc, it is difficult to attach the cap to the optical disc. In addition, the optical disc can be lifted when the cap is detached from the optical disc after spin coating.
According to U.S. Pat. No. 6,689,415, a center hole of an optical disc is covered by a cap, which has a holding axis in its central portion. By using the holding axis, the cap can be attached to and detached from the optical disc. In this case, however, when the cap is eccentrically placed on the optical disc, it is difficult to attach the cap to the optical disc. In addition, the optical disc can be lifted when the cap is detached after spin coating.